Noctem
Explorer
It says it in the rules and the spell. It says used Cast a Spell action for casting any spell, even it requires an attack roll. A spell like scorching ray says you get fire up to three fire rays from one casting. You don't get to use them next round. You don't get to pick the target after you know the result of the first hit. It says you get to fire three rays that require attack rolls. That means you exactly what it says. Pick targets, fire three rays, roll attack rolls, resolve effect.
Just as you used an example like magic missile to say they must say that to fire simultaneously, I'll pick a spell like sunbeam to show what a spell looks like that gives multiple beams but allows you to use it over multiple rounds and targets.
The spell will tell you if you get to wait for the attacks to resolve before picking targets. If the spell does not clearly tell you to fire a ray, see if it hits, then pick targets, you have to pick them at all at once. It's an implied part of the Cast a Spell action. Nowhere in eldritch blast or scorching ray does it say, "Fire ray, see what happens, fire next ray, see what happens.". The spell generates the rays or beams when you cast it. Period. It does nothing else. To see if the rays hit you make an attack roll. That is a property of the ray, not the spell. The effect of the spell is to generate all the rays. You have to pick targets when you cast the spell because the spell generates the rays when you cast it. Unlike swinging a sword which lets you pick a target each swing.
You're looking at the spell as giving you the ability to generate rays when you want to do so such as after the attack is resolved. That is not what eldritch blast or scorching ray do. The spell generates the rays and you have to know how you're doing to use them immediately when you cast the spell because the rays are all generated immediately when the spell is cast...otherwise known as simultaneously. It doesn't say they pulse or go off 1 second after another or you can walk between beams or any text of the kind. It says you generate the rays and make the hit rolls. No other actions taken between. You can pick targets within range. They must be picked when you cast it with no other decision as part of the process other than casting the spell and available targets within range. You don't get to see damage done. You don't get to fire a beam, move, and pick another target. You don't get to fire two beams and ready a third beam when a target gets in range. You fire them all at once when you cast the spell.
I will strongly recommend that you stop posting nonsense and go actually read the rules you're trying to discuss. You're adding a bunch of stuff to the rules text in the PHB in an attempt to make a coherent statement. The problem is that as soon as someone actually go to read the rules, they immediately see that you have no idea what you're talking about. That is of course unless you realize that you're forwarding a houserule that you use at home and aren't trying to convince others that your houserule follows the rules in the book. Stop spreading misinformation please.